Diffusivity and solubility of hydrogen as a function of composition in Fe-Ni alloys
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J. O'M.
BOCKRIS,
M. A. GENSHAW,
AND
P. K. SUBRAMANYAN
A n e l e c t r o c h e m i c a l method has b e e n u s e d to d e t e r m i n e the p e r m e a b i l i t y , P, diffusion coe f f i c i e n t , D , and s o l u b i l i t y , c , of h y d r o g e n in a l l o y s of the F e - N i s y s t e m . The heats of a c t i v a t i o n for diffusion and the h e a t s of s o l u t i o n have b e e n d e r i v e d . D f a l l s f r o m ---10 -4 sq c m p e r sec f o r p u r e i r o n to ~ 1 0 -1~ sq c m p e r sec for 40 wt pct of Ni in the a l l o y . T h e r e a f t e r it r i s e s s l i g h t l y to that for p u r e n i c k e l , c r i s e s by about 103 b e t w e e n p u r e i r o n and 40 wt pct Ni, then r e m a i n s c o n s t a n t up to p u r e n i c k e l . The r e s u l t a n t P d o u b l e s at 5 wt pct Ni and then f a l l s by 103 t i m e s up to 40 wt pct Ni, a f t e r w a r d s r i s i n g s l i g h t l y to that for p u r e n i c k e l . B e t w e e n 0 and 40 wt pct Ni the d o m i n a n t f a c t o r in c o n t r o l l i n g the v a l u e of P is the fall of the m o l e f r a c t i o n of the ot p h a s e in the a l l o y . T h i s h y p o t h e s i s g i v e s a r e a s o n a b l e q u a n t i t a t i v e c a l c u l a t i o n of the P - c o m p o s i t i o n r e l a t i o n . B e t w e e n 40 and 100 wt pct, the c r y s t a l l o g r a p h i c p h a s e is a l l 7 and the m a j o r effect is the bonding of hyd r o g e n in the alloy, the s m a l l c h a n g e s noted b e i n g r e a s o n a b l y c a l c u l a b l e . The n e g l i g i b l e change of s o l u b i l i t y in this r e g i o n r e f l e c t s the n e g l i g i b l e change in d c h a r a c t e r of the a l loy f r o m 40 to 100 wt pct Ni. The h y d r o g e n p e r m e a b i l i t y of F e - N i (5 wt pct) is g r e a t e r than that of p a l l a d i u m at T > 200~ The c o r r o s i o n r a t e and h y d r o g e n p e r m e a b i l i t y (hence, s u s c e p t i b i l i t y to h y d r o g e n e m b r i t t l e m e n t ) p a s s t h r o u g h a m i n i m u m at about 50 wt pct Ni. A r e m a r k a b l e p a r a l l e l i s m e x i s t s b e t w e e n c o r r o s i o n r a t e and h y d r o g e n p e r m e a t i o n in F e - N i a l l o y s . An i n t e r p r e t a t i o n is s u g g e s t e d .
T H E study of h y d r o g e n p e r m e a t i o n t h r o u g h F e - N i a l loys is i n t e r e s t i n g f r o m both t e c h n i c a l and a c a d e m i c v i e w p o i n t s . The diffusivity of h y d r o g e n in p u r e bcc i r o n is known to be the h i g h e s t of a l l m e t a l s . However, the s o l u b i l i t y in bcc i r o n is v e r y s m a l l . On the o t h e r hand, the s o l u b i l i t y of h y d r o g e n in n i c k e l is m u c h l a r g e r t h a n in i r o n , but the d i f f u s i v i t y is s m a l l e r by t h r e e o r d e r s of m a g n i t u d e . P e r m e a t i o n study through a c o m p l e t e s e r i e s of F e - N i a l l o y s , including the end c o m p o n e n t s , is i m p o r t a n t for e v a l u a t i
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